Such fuel tanks are commonly mounted underground, and filled by gravity feed delivery tanker trucks. Delivery flow rates vary over a wide range depending on the tanker compartment fuel level and delivery pipework size and length. In some cases, tanker pipework delivery is assisted by a pump system to reduce delivery time, and the flow rate may vary from approximately 200 to approximately 2,000 liters per minute. At high delivery rates, considerable turbulence can occur below the end of the fill tube, and accordingly it has been proposed to incorporate a diffuser, at the lower end of the fill tube, which is arranged to deflect the flow of fuel in a horizontal direction, instead of allowing it to flow vertically downwards towards the base of the tank.
One known type of diffuser of this kind is shown in British patent no. 2,344,582, and comprises a tubular part which is attached to the lower end of the fill tube, having slot-like apertures in its side walls, and a base which includes a conical shaped deflector. In use, the downwardly flowing fuel is deflected by the sloping sides of the cone so as to leave the diffuser through the slot-like apertures.
In some applications however, operators wish to check the level of fuel in the tank, and also for the presence of water, by inserting a dipstick into the fill tube. As the diffuser is deliberately positioned above the bottom of the tank, the presence of the diffuser will prevent a proper reading of the depth being obtained. In practice, since tanks vary in size, the fill tube/diffuser combination is made to be somewhat shorter than the expected depth of the tank, preventing the effective use of such a dipstick.